1. Technical Field
The present invention relates to a pneumatic tire, and particularly relates to a pneumatic tire having a sipe formed on a land portion of a tread portion.
2. Background Art
A land portion such as a block or a rib is provided on a tread portion of a pneumatic tire, a cut having a narrow groove width called a sipe is sometimes provided on the land portion. Formation of the sipe increases running performance and braking performance on a snow and ice road surface and the like by its edge effect and water removal effect. Generally, the sipes are formed at a constant groove width in its length direction. In such a case, the sipes are uniformly closed by, for example, a backward and forward force loaded at the time of braking, and this gives rise to the problem that the inherent edge effect of the sipe is decreased.
JP-A-09-272312 (1997) proposes that a wide part having a wide groove width from a tread surface to a sipe bottom is provided on at least three places in a length direction of the sipe. Formation of such a wide part prevents the sipe from closing as a whole, thereby the edge effect can be improved. However, in JP-A-09-272312, all of the wide parts extend from the tread surface to the sipe bottom. This structure gives rise to the problem that rigidity difference between a stepping side and a kicking side is large in the wide part, and a toe and heel wear amount is large.
JP-A-2005-247105 proposes that a first wide part which extends from a tread surface in a depth direction and is terminated in the halfway to a bottom of a sipe and a second wide part which extends from almost the same height as the bottom of the first wide part to the sipe bottom are provided on the sipe. JP-A-2010-089541 proposes that a wide part which has an inclined plane inclined such that a groove width is decreased toward a sipe bottom side in an opening of the sipe is alternately provided on both sipe walls along a length direction of the sipe. Those structures can prevent the sipe as a whole from closing in at least an initial stage of wear. However, the structures are not a structure having rigidity difference such that the rigidity is increased stepwise in the length direction of the sipe. Therefore, the structures are difficult to achieve both the edge effect and the toe and heel wear suppressing effect, similar to the above JP-A-09-272312.